1.) Field of the Invention
This invention relates generally to apparatus for logging while drilling and more particularly to logging while drilling tools which include gamma ray scintillation detectors for measuring the natural radioactivity of geological formations surrounding a borehole.
2.) Description of the Prior Art
Gamma ray sensing instruments including a sodium iodide (NaI) crystal in combination with a photo multiplier tube (PMT) have been used for many years to sense and count the number of gamma rays striking a measurement tool in a borehole. Such instruments have been used with great success in open hole logging tools to measure the extent of natural radioactivity of earth formations which surround the borehole. When such gamma ray sensing instruments are used in a logging while drilling (LWD) environment, however, tremendous shocks are imparted to the housing of the LWD tools. Such shocks can induce voltage pulses from the detector that have pulse amplitudes similar to the amplitudes generated by gamma rays striking the detector.
The art of engineering shock absorbing mechanical mountings for NaI crystals and PMT's has found that it is difficult and expensive to build gamma ray detectors which limit shock induced voltage pulses to a few pulses or "counts" per shock. Furthermore, experience teaches that a gamma ray detector may exhibit low shock induced count rates when it is first put in service; but, after field use, the shock count rate increases.